The disclosure generally relates to compounds of formula I, including their salts, as well as compositions and methods of using the compounds. The compounds inhibit GSK-3 and may be useful for the treatment of various disorders of the central nervous system.
GSK-3 is a proline directed serine/threonine kinase that carries out the phosphorylation of multiple protein substrates. Many of these proteins are involved in the regulation of numerous diverse cellular functions, including metabolism, differentiation, proliferation and apoptosis. GSK-3 is constitutively active, with its base level of activity being positively modulated by phosphorylation on Tyr216/219, depending on isoform. GSK-3 has a unique substrate selectivity profile that is distinguished by the strong preference for the presence of a phosphorylated residue optimally located four amino acids C-terminal to the site of GSK-3 phosphorylation. Most commonly, GSK-3 activity is associated with inducing a loss of substrate function, such that GSK-3 inhibition will frequently result in increased downstream substrate activity.
GSK-3 exists in two isoforms, GSK-3α (51 kDa) and GSK-3β (47 kDa), that share 84% overall identity and greater than 98% identity within their respective catalytic domains. Both primary isoforms are ubiquitously expressed, with high levels observed in the brain, particularly in the cortex and hippocampus. In most brain areas, GSK-3β is the predominant isoform. However, some studies suggest that GKS-3α and GSK-3β share very similar, if not entirely redundant functions in a number of cellular processes. The activity of GSK-3β is significantly reduced by phosphorylation at Ser9 in the N-terminal domain, most notably by protein kinase B (PKB or AKT). This inhibitory pathway has been proposed to result in neuroprotection, neurogenesis, and favorable outcomes following pharmacological treatment in various mood disorders.
Alzheimer's disease (AD) pathology is prominently associated with the formation of beta-amyloid (Aβ) plaques, soluble forms of Aβ such as Aβ1-42 that are associated with increased neuronal toxicity, and neurofibrillary tangles (NFTs). There is evidence to suggest that certain pathological mechanisms in AD, such as Aβ1-42, cause increases in GSK-3 activity in the brain. A principal consequence of this dysregulation is the hyperphosphorylation of the microtubule associated protein tau. This function of GSK-3 has been demonstrated both in cell culture, and in in vivo studies looking at tau and NFT formation. Hyper-phosphorylated tau disengages from microtubules resulting in structural destabilization of microtubules with concomitant negative effects on intracellular structures and transport mechanisms. In addition, the uncomplexed hyperphosphorylated tau assembles into paired helical filaments (PHFs) that aggregate to produce the stereotypic intracellular NFTs associated with AD. Other potential pathological consequences of over-activation of GSK-3 include neuroinflammation and neuronal apoptosis. In addition, GSK-3 has been demonstrated to be involved in mechanisms underlying memory and learning, and dysregulation of GSK-3 function may explain some of the early cognitive deficits observed in AD.
GSK-3 is also known to play a key role in glucose metabolism, and was first identified as the enzyme responsible for effecting the inhibitory phosphorylation of glycogen synthase, the result of which is to reduce the rate of conversion of glucose to glycogen, giving rise to elevated blood glucose levels. This function of GSK-3 is controlled by insulin. Binding of insulin to its receptor leads indirectly to the activation of AKT and subsequent inhibitory Ser9 phosphorylation of GSK-3.
These results and observations suggest that modulation of GSK-3 activity may be useful in the treatment of both the neuropathologic and symptomatic aspects of Alzheimer's disease, as well as other neurodegenerative diseases. These include, but are not limited to, tauopathies (for example, frontotemporal dementia, progressive supranuclear palsy, argyophilic grain disease, corticobasal degeneration, Pick's disease), Parkinson's disease, amyotrophic lateral sclerosis, stroke, Huntington's disease, peripheral neuropathies, traumatic brain injury, spinal cord trauma, and vascular dementias.
Compounds that inhibit GSK-3 may also have utility in the treatment of diabetes, inflammatory diseases such as rheumatoid arthritis and osteoarthritis, treatment-resistant depression, schizophrenia, bipolar disorder, manic depression, osteoporosis, cardioprotection, and various cancers such as gliomas, non-small cell lung cancer, pancreatic cancer, breast cancer, T- or B-cell leukemia, and multiple myeloma.
Recent reviews on the functions of GSK-3, potential therapeutic applications, and other compounds that inhibit the enzyme are listed below:    Kaidanovich-Beilin O and Woodgett J R (2011) GSK-3: functional insights from cell biology and animal models. Front. Mol. Neurosci. 4:40. doi: 10.3389/fnmol.2011.00040.    “Glycogen Synthase Kinase 3 (GSK-3) and Its Inhibitors”, Martinez, Ana/Castro, Ana/Medina, Miguel (eds.), John Wiley and Sons (2006).    Gentles, R G, Hu, S. and Dubowchik, G M (2009) Recent Advances in the Discovery of GSK-3 Inhibitors and a Perspective on their Utility for the Treatment of Alzheimer's Disease. Annual Reports in Medicinal Chemistry 44, 3-26.
The invention provides technical advantages, for example, the compounds are novel inhibitors of GSK-3 and may be useful for the treatment of various disorders of the central nervous system. Additionally, the compounds provide advantages for pharmaceutical uses, for example, with regard to one or more of their mechanism of action, binding, inhibition efficacy, target selectivity, solubility, safety profiles, or bioavailability.